1. Field of Invention
This invention relates to a system for determining the absolute time of the occurrence of events at one or more spaced locations using a local counter at each location and a single, remote absolute clock, and has particular application to time stamping contact activity in a number of protective relays in an electric power distribution network.
2. Background Information
There are many applications where it is desirable to record the real time, known as time stamping, of certain events to provide a historic view of the activity of a particular device. Normally, a real-time clock is used to provide accurate time references for the stamps. Difficulties arise, however, when several devices are linked and yet do not share the same clock reference. Clocks will drift with respect to each other resulting in inaccurate time stamps.
This problem can occur in attempting to time stamp contact activity in protective relays which control circuit breakers in an electric power distribution system. It is desirable in such an application to determine the time of contact activity in the different relays with an accuracy of a few milliseconds for use in analyzing the sequence of events if there is a disturbance in the system.
It is an object of the invention, therefore, to provide a system which generates accurate time stamps of events in a number of devices such as contact activity in the protective relays in an electric power transmission system.
It is a more specific object of the invention to provide such a system which utilizes a single absolute clock to provide accurate time stamps for monitored events in each of the separate devices.
It is another object of the invention to provide such a system which provides accurate time stamps for at least a certain number of the most recent of the monitored events in each device.
It is still another object of the invention to provide such a system which stores information used to generate the time stamps in slave stations associated with each device and transmits this information to a master station which contains the single absolute clock.